Traction distribution control system for four-wheel drive vehicle

ABSTRACT

A traction distribution control system for a 4WD vehicle is so constructed as to carry out traction distribution control to increase distribution of traction to the driven wheels when a speed difference between the main driving wheels and the driven wheels exceeds a control threshold value, detect constant-speed run of the vehicle, and carry out correction control to correct the control threshold value in accordance with the wheel-speed difference when constant-speed run is detected.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a traction distribution control system for a four-wheel drive (4WD) vehicle, which distributes traction of a power source to a pair of main driving wheels or one of pairs of front wheels and rear wheels and a pair of driven wheels or another pair of wheels.

[0002] Typically, a traction distribution control system for a 4WD vehicle comprises engaging means arranged in a path transferring power of an engine to the main driving wheels and for transferring power to the driven wheels when the engaging means are in engagement, wherein the engagement state of the engaging means is controlled to control traction distribution to the driven wheels.

[0003] Traction distribution in the traction distribution control system is typically carried out such that a difference is obtained between an average speed of the front wheels and an average speed of the rear wheels, wherein as the difference is greater, engaging force of the engaging means is increased to enhance the ratio of traction distribution to the driven wheels, and wherein as the difference is smaller, engaging force of the engaging means is decreased to lower the ratio of traction distribution to the driven wheels.

[0004] With the above traction distribution control system, when the main driving wheel and the driven wheel are provided with tires having different diameters, a difference in rotation speed occurs between the main driving wheel and the driven wheel even though the main driving wheel and the driven wheel at the outer peripheries rotate at the same speed. Then, it is determined that the smaller-diameter wheel causes a slip, so that engaging force of the engaging means is changed to provide traction to the larger-diameter wheel, raising problems of degradation of the durability of a power train, occurrence of vibrations, decrease in fuel consumption, etc.

[0005] In order to solve such problem, an apparatus is proposed wherein mounting of a different-diameter tire is detected to control engaging force of the engaging means. Specifically, a tire-diameter difference is determined in accordance with average vehicle speed and average clutch torque. And engaging force of a clutch is determined in accordance with smaller one of a clutch-torque calculation result wherein as the determined tire-diameter difference is larger and as the vehicle speed is higher, a dead zone of clutch engagement is increased, and a maximum clutch-torque calculation result wherein as the vehicle speed is higher, the dead zone is decreased. This allows selection of appropriate clutch engaging force in response to the vehicle speed, resulting in enhancement in the durability of a power train including a transfer, a differential, etc., and prevention of vibrations and poor fuel consumption.

[0006] With the above apparatus, a tire-diameter difference is determined in accordance with average vehicle speed and average clutch torque, which needs, however, complicated and accurate control, leading to a cost increase of the apparatus. Further, an area where clutch engaging force does not occur, i.e. dead zone, is determined in accordance with a rotation-speed difference between the driven wheels and the main driving wheels, one of which is equipped with a different-diameter tire. That is, the dead zone is set to be larger as a wheel-speed difference is greater. Furthermore, one of clutch engaging torques determined in accordance with the rotation-speed difference is not selected, which results from rotation-speed difference between the driven wheel and the main driving wheel, which is equipped with a different-diameter tire. Thus, appropriate traction cannot be distributed to the driven wheels in accordance with the vehicle running conditions, resulting in deterioration of vehicle start-ability and stability.

SUMMARY OF THE INVENTION

[0007] It is, therefore, an object of the present invention to provide a traction distribution control system for a 4WD vehicle, which allows appropriate traction distribution to the driven wheels in accordance with the vehicle running conditions with simple control, thus contributing to a cost reduction of the system and an enhancement in vehicle start-ability and stability.

[0008] The present invention provides generally a four-wheel drive vehicle, which comprises: a source of traction; a main path which serves to transfer traction; an auxiliary path branched from the main path, the auxiliary path serving to transfer traction; main driving wheels which receive traction from the source through the main path; driven wheels which receive traction through the auxiliary path; a clutch arranged in the auxiliary path, the clutch changing traction to be transferred to the driven wheels through a change in engaging force; a device which detects a speed of a vehicle body; a device which detect speeds of the respective wheels; and an ECU which provides to the clutch a control signal for carrying out the change in engaging force, wherein the ECU is so programmed as to: calculate a control threshold value in accordance with the detected vehicle-body speed, carry out a traction distribution control to increase distribution of traction to the driven wheels when a difference in detected wheel speeds between the main driving wheels and the driven wheels exceeds the control threshold value, detect a constant-speed run of the vehicle, and carry out a correction control to correct the control threshold value in accordance with the difference in detected wheel speeds when the constant-speed run is detected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The other objects and features of the present invention will become apparent from the following description with reference to the accompanying drawings, wherein:

[0010]FIG. 1 is a block diagram showing a traction distribution control system for a 4WD vehicle, to which the present invention is applied;

[0011]FIG. 2 is a flowchart showing the operation of an embodiment of the present invention;

[0012]FIG. 3A is a table showing the offset-gain characteristic;

[0013]FIG. 3B is a graph illustrating the offset-gain characteristic; and

[0014] FIGS. 4A-4B are graphs similar to FIG. 3B, each illustrating a variation in a wheel-speed offset-gain value.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Referring to the drawings, a description is made about a traction distribution control system for a 4WD vehicle embodying the present invention.

[0016] Referring to FIG. 1, the 4WD vehicle to which the present invention is applied is a front-drive vehicle having front wheels as main driving wheels and rear wheels as driven wheels. Traction of an engine 1 is transmitted to a transfer 3 through an automatic transmission 2 having a torque converter, which is distributed to a front propeller shaft 4 and a rear propeller shaft 5 by transfer 3. Traction transmitted to front propeller shaft 4 is always transferred to front wheels WFR, WFL through a front differential 6. A traction transfer path extending from engine 1 to front wheels WFR, WFL serves as a main traction transfer path. On the other hand, traction transmitted to rear propeller shaft 5 is transferred to rear wheels WRR, WRL through a clutch or traction distribution means 7 and a rear differential 8. A traction transfer path extending from rear propeller shaft to rear wheels WRR, WRL serves as an auxiliary traction transfer path.

[0017] Thus, in the state that clutch 7 is in full disengagement, the ratio of traction distribution between front wheels WFR, WFL and rear wheels WRR, WRL is 100:0, whereas in the state that cutch 7 is in full engagement, the ratio is 50:50. Therefore, by changing the engaging state of clutch 7, traction distribution between wheels WFR, WFL and rear wheels WRR, WRL can arbitrarily be changed in the range of 100:0 to 50:50.

[0018] The engaging state of clutch 7 is changed in accordance with the magnitude of a control output current I derived from an electronic control unit (ECU) or traction distribution control means 10. ECU 10 is connected to wheel-speed sensors 11 for sensing speeds Vw of wheels WFR, WFL, WRR, WRL. When required, ECU 10 may be connected to a longitudinal acceleration sensor 12 for sensing longitudinal acceleration of the vehicle and a lateral acceleration sensor 13 for sensing lateral acceleration of the vehicle. Sensors 11-13 can serve as sensors arranged in an ABS control system for preventing wheel lock during braking.

[0019] Referring to FIG. 2, traction distribution control and correction control carried out by ECU 10 are described. At steps S201-S206, processing is carried out to determine whether or not the vehicle runs at constant speed, and obtain a wheel-speed offset value VOFST in accordance with a current vehicle-body speed Vc.

[0020] At step S201, current vehicle-body speed Vc is stored in a storage, not shown, and is designated as a previous vehicle-body speed Vcm. Previous vehicle-body speed Vcm is updated every control cycle. In this embodiment, vehicle-body speed Vc is obtained from a speed Vw of each wheel detected by wheel-speed detecting means. Wheel-speed sensor 11 and part of ECU 10 for obtaining the vehicle-body speed in accordance with a signal out of wheel-speed sensor 11 constitute vehicle-body speed detecting means. When obtaining vehicle-body speed Vc, the use of a signal out of longitudinal acceleration sensor 12 can enhance the detection accuracy. Optionally, the vehicle-body speed detecting means may comprise a vehicle-speed sensor for sensing the rotation speed of the main traction transfer path including front propeller shaft 4.

[0021] At step S202, it is determined whether or not the vehicle runs at constant speed. Specifically, it is determined whether or not an absolute value of a difference between current vehicle-body speed Vc and previous vehicle-body speed Vcm is smaller than a predetermined set value 3.5 km/h. If it is determined that |Vc−Vcm|<3.5 km/h, i.e. the vehicle runs at constant speed, flow proceeds to step S203 where a timer for determining constant-speed run is incremented by 1, and previous vehicle-body speed Vcm is updated to current vehicle-body speed Vc. On the other hand, at step S202, it is determined that that |Vc−Vcm|=3.5 km/h, i.e. the vehicle does not run at constant speed, flow proceeds to step S204 where the timer is cleared, and previous vehicle-body speed Vcm is updated to current vehicle-body speed Vc. The set value, which is a value for determining constant-speed run, is not limited to 3.5 km/h since it is set allowing for a variation in vehicle speed which will occur even if a driver intends to cruise his/her vehicle at constant speed.

[0022] At step S205, wheel-speed offset value VOFST as a control threshold value is calculated by:

VOFST=Vc×(GVOFST/Vcm2)

[0023] where GVOFST is an offset gain, and Vcm2 is a stored vehicle-body speed as will be described later. Wheel-speed offset value VOFST is a value serving as a reference for determining whether or not traction distribution is carried out to rear wheels WRR, WRL as driven wheels (refer to a step S212), and corresponds to about 1% of vehicle-body speed Vc.

[0024] At step S206, it is determined whether or not the timer for determining constant-speed run exceeds a predetermined set time of 10 sec. If it is determined that the timer exceeds the set time, the vehicle is determined to run at constant speed, and correction control at steps S207-S211 is carried out. On the other hand, if it is determined that the timer does not exceed the set time, the vehicle is determined to run at non-constant speed, and flow proceeds to step S212 directly.

[0025] With correction control, at step S207, greater one of a value obtained by subtracting rear-wheel speed VWR (e.g. VWR=(VWRL+VwRL)/2) from front-wheel speed VWF (e.g. VWF=(VWFL+VwFL)/2) and zero is set as a speed difference DELTV between the front and rear wheels. Specifically, wheel-speed difference DELTV is a value for determining the degree of slip of front wheels WFR, WFL as main driving wheels.

[0026] At step S208, a rotation-difference comparative value obtained by subtracting 1% of vehicle-body speed Vc from wheel-speed difference DELTV is compared with wheel-speed offset value VOFST. If the comparative value is equal to wheel-speed offset value VOFST, flow proceeds to step S211. If the comparative value is greater than wheel-speed offset value VOFST, flow proceeds to step S210 where offset gain GVOFST is incremented, whereas if it is smaller than VOFST, flow proceeds to step S209 where offset gain GVOFST is decremented. Specifically, offset gain GVOFST is set so that wheel-speed offset value VOFST converges to 1% of vehicle-body speed Vc. When wheel-speed difference DELTV during constant-speed run is greater than 1% of vehicle-body speed Vc, offset gain GVOFST is increased. When wheel-speed difference DELTV is smaller than 1% of vehicle-body speed Vc, offset gain GVOFST is decreased. And when wheel-speed difference DELTV is equal to 1% of vehicle-body speed Vc, offset gain GVOFST is maintained as it is.

[0027] At step S211 to which flow proceeds after setting of offset gain GVOFST at steps S208-S210, vehicle-body speed Vc at a point that offset gain GVOFST is formed is updated as stored vehicle-body speed Vcm2. Offset gain GVOFST decremented or incremented at step S209 or S210 is stored in the storage until next correction control, i.e. processing at step S209 or S210 is carried out in accordance with determination at step S208. Moreover, stored vehicle-body speed Vcm2 is held as it is until offset gain GVOFST is updated. The above processing corresponds to preservation of a correction result.

[0028] At step S212, control torque TDV for a rotation difference between the front and rear wheels is obtained by:

[0029] ti TDV=max (0, (DELTV−VOFST)−k)

[0030] where k is a constant. That is, rotation-difference control torque TDV is formed such that when wheel-speed difference DELTV exceeds wheel-speed offset value VOFST as a control threshold value, torque proportional to a value corresponding to a difference therebetween is distributed to rear wheels ERR, ERL as driven wheels. At a subsequent step S213, a control output current I corresponding to rotation-difference control torque TDV is provided to clutch 7.

[0031] The operation of this embodiment is described. In this embodiment, if it is determined that a variation in vehicle-body speed Vc is smaller than the set value of 3.5 km/h, i.e. the vehicle runs at constant speed, correction control is carried out to form offset gain GVOFST in accordance with wheel-speed difference DELTV produced at that time. Specifically, during constant-speed run, the front and rear wheels should rotate at the same speed. However, if wheel-speed sensors 11 provide different detection values between the front and rear wheels, this shows that a diameter difference occurs between the tires of the front and rear wheels. This tire-diameter difference can occur not only when the front and rear wheels are equipped with different tires, but also when wear of the front wheels as main driving wheels develops with respect to that of the rear wheels as driven wheels after a long time of use of the tires without rotation.

[0032] If wheel-speed difference DELTV occurs during constant-speed run, DELTV is compared with 1% of vehicle-body speed Vc as a set value. Referring to FIG. 3A, when DELTV=Vc×0.01, offset gain GVOFST is maintained as it is. When DELTV>Vc×0.01, correction is carried out to increase offset gain GVOFST. And when DELTV<Vc×0.01, correction is carried out to decrease offset gain GVOFST. Referring to FIG. 3B, this changes an inclination of offset gain GVOFST.

[0033] Therefore, when increasing offset gain GVOFST, wheel-speed offset value VOFST calculated at step S205 becomes relatively great, widening the dead zone. On the other hand, when decreasing wheel-speed offset value VOFST calculated at step S205 becomes relatively small, narrowing the dead zone. Referring to FIGS. 4B-4B, the dead zone is an area between wheel-speed offset value VOFST indicated by broken line and wheel-speed difference DELTV=0, wherein FIG. 4A shows dead zone narrowed by decreasing offset gain GVOFST, and FIG. 4B shows dead zone widened by increasing offset gain GVOFST. In such a way, wheel-speed difference DELTV has a value within the dead zone, traction T is not distributed to rear wheels WRR, WRL.

[0034] Then, when a difference between wheel-speed difference DELTV and 1% of vehicle-body speed Vc is greater than wheel-speed offset value VOFST during constant-speed run (when the answer at step S206 is YES), and it is thus determined that a tire-diameter difference is greater (flow of step S207 step S208 step S210), correction is carried out to increase offset gain GVOFST. With this, even if the dead zone is widened as shown in FIG. 4B, and wheel-speed difference DELTV occurs due to tire-diameter difference, wheel-speed offset value VOFST calculated at step S205 is not affected by the tire-diameter difference. Moreover, in this state, when the speed of front wheels WFR, WFL as main driving wheels becomes higher, and wheel-speed difference DELTV becomes greater than an amount corresponding to a tire-diameter difference to exceed wheel-speed offset value VOFST, traction is distributed to rear wheels WRR, WRL.

[0035] On the other hand, when wheel-speed difference DELTV is small during constant-speed run, and it is thus determined that a tire-diameter difference is small, correction is carried out to decrease offset gain GVOFST as shown in FIG. 4B, narrowing the dead zone, allowing prevention of degradation of the control responsivity when carrying out traction distribution control to rear wheels WRR, WRL. An increase or decrease in offset gain GVOFST is carried out only once during one control cycle, but again and again until a difference between wheel-speed difference DELTV and 1% of vehicle-body speed Vc converges to wheel-speed offset value VOFST, providing excellent convergence with less occurrence of control hunting.

[0036] As described above, in the embodiment, wheel-speed offset value VOFST is set to an optimum value in accordance with a tire-diameter difference so as to prevent not only occurrence of heat and vibrations caused by execution of needless traction distribution due to a tire-diameter difference, but also degradation of the control responsivity due to excessive wheel-speed offset value VOFST, allowing compatibility between the vehicle start-ability and the vehicle stability. And when increasing or decreasing wheel-speed offset value VOFST to an optimum value to obtain such effect, detection of constant-speed run is carried out, and offset gain GVOFST is set in accordance with wheel-speed difference DELTV at that time, requiring only simple comparison as control operation, allowing simplified control as compared with the prior art, resulting in achievement of the above effect with low-priced construction. Moreover, offset gain GVOFST and stored vehicle-body speed Vcm2 or vehicle-body speed Vc when offset gain GVOFST is updated are stored in the storage until next correction control is carried out, allowing reflection of this result on next traction distribution control, resulting in excellent control responsivity.

[0037] Having described the present invention with regard to the illustrative embodiments, it is noted that the present invention is not limited thereto, and various changes and modifications can be made without departing from the scope of the present invention. By way of example, the present invention is applied to a vehicle having front wheels as main driving wheels, alternately, it can be applied to a vehicle having rear wheels as main driving wheels. Further, in the illustrative embodiment, traction distribution between front wheels WFR, WFL and rear wheels WRR, WRL is carried out in the range of 100:0 to 50:50, alternatively, it can be changed in a narrower range.

[0038] The entire contents of Japanese Patent Application P2002-034414 filed Feb. 12,2002 are hereby incorporated by reference. 

What is claimed is:
 1. A four-wheel drive vehicle, comprising: a source of traction; a main path which serves to transfer traction; an auxiliary path branched from the main path, the auxiliary path serving to transfer traction; main driving wheels which receive traction from the source through the main path; driven wheels which receive traction through the auxiliary path; a clutch arranged in the auxiliary path, the clutch changing traction to be transferred to the driven wheels through a change in engaging force; a device which detects a speed of a vehicle body; a device which detect speeds of the respective wheels; and an ECU which provides to the clutch a control signal for carrying out the change in engaging force, the ECU being so programmed as to: calculate a control threshold value in accordance with the detected vehicle-body speed, carry out a traction distribution control to increase distribution of traction to the driven wheels when a difference in detected wheel speeds between the main driving wheels and the driven wheels exceeds the control threshold value, detect a constant-speed run of the vehicle, and carry out a correction control to correct the control threshold value in accordance with the difference in -detected wheel speeds when the constant-speed run is detected.
 2. The four-wheel drive vehicle as claimed in claim 1, wherein a result of the correction control is stored in a storage until a next correction control is carried out.
 3. The four-wheel drive vehicle as claimed in claim 1, wherein the correction control is carried out such that: when a rate of an absolute value of the difference in detected wheel speeds to the vehicle-body speed is larger, it is determined that a difference in diameter between the main driving wheels and the driven wheels is greater, and then the control threshold value is increased gradually; and when the rate of the absolute value of the difference to the vehicle-body speed is smaller, it is determined that the difference in diameter is smaller, and then the control threshold value is decreased gradually.
 4. The four-wheel drive vehicle as claimed in claim 1, wherein the control threshold value is calculated such that the vehicle-body speed is multiplied by an offset gain, and then divided by another vehicle-body speed upon calculation of the offset gain.
 5. The four-wheel drive vehicle as claimed in claim 4, wherein the correction control is carried out by setting the offset gain in accordance with a relationship between the difference in detected wheel speeds and the vehicle-body speed, wherein the control threshold value is increased gradually by increasing gradually the offset gain, and the control threshold value is decreased gradually by decreasing gradually the offset gain.
 6. The four-wheel drive vehicle as claimed in claim 1, wherein the traction distribution control is carried out by determining the engaging force of the clutch in accordance with a difference between the difference in detected wheel speeds and the control threshold value.
 7. A four-wheel drive vehicle, comprising: a source of traction; a main path which serves to transfer traction; an auxiliary path branched from the main path, the auxiliary path serving to transfer traction; main driving wheels which receive traction from the source through the main path; driven wheels which receive traction through the auxiliary path; a clutch arranged in the auxiliary path, the clutch changing traction to be transferred to the driven wheels through a change in engaging force; means for detecting a speed of a vehicle body; means for detecting speeds of the respective wheels; and means for providing to the clutch a control signal for carrying out the change in engaging force, the providing means comprising: means for calculating a control threshold value in accordance with the detected vehicle-body speed, means for carrying out a traction distribution control to increase distribution of traction to the driven wheels when a difference in detected wheel speeds between the main driving wheels and the driven wheels exceeds the control threshold value, means for detecting a constant-speed run of the vehicle, and means for carrying out a correction control to correct the control threshold value in accordance with the difference in detected wheel speeds when the constant-speed run is detected.
 8. The four-wheel drive vehicle as claimed in claim 7, wherein a result of the correction control is stored in a storage until a next correction control is carried out.
 9. The four-wheel drive vehicle as claimed in claim 7, wherein the correction control is carried out such that: when a rate of an absolute value of the difference in detected wheel speeds to the vehicle-body speed is larger, it is determined that a difference in diameter between the main driving wheels and the driven wheels is greater, and then the control threshold value is increased gradually; and when the rate of the absolute value of the difference to the vehicle-body speed is smaller, it is determined that the difference in diameter is smaller, and then the control threshold value is decreased gradually.
 10. The four-wheel drive vehicle as claimed in claim 7, wherein the control threshold value is calculated such that the vehicle-body speed is multiplied by an offset gain, and then divided by another vehicle-body speed upon calculation of the offset gain.
 11. The four-wheel drive vehicle as claimed in claim 10, wherein the correction control is carried out by setting the offset gain in accordance with a relationship between the difference in detected wheel speeds and the vehicle-body speed, wherein the control threshold value is increased gradually by increasing gradually the offset gain, and the control threshold value is decreased gradually by decreasing gradually the offset gain.
 12. The four-wheel drive vehicle as claimed in claim 7, wherein the traction distribution control is carried out by determining the engaging force of the clutch in accordance with a difference between the difference in detected wheel speeds and the control threshold value.
 13. A method of controlling traction distribution in a four-wheel drive vehicle with a traction source, main driving wheels, driven wheels, and a clutch, the method comprising: detecting a speed of a vehicle body; detecting speeds of the respective wheels; providing to the clutch a control signal for carrying out the change in engaging force; calculating a control threshold value in accordance with the detected vehicle-body speed; carrying out a traction distribution control to increase distribution of traction to the driven wheels when a difference in detected wheel speeds between the main driving wheels and the driven wheels exceeds the control threshold value; detecting a constant-speed run of the vehicle; and carrying out a correction control to correct the control threshold value in accordance with the difference in detected wheel speeds when the constant-speed run is detected.
 14. The method as claimed in claim 13, wherein a result of the correction control is stored in a storage until a next correction control is carried out.
 15. The method as claimed in claim 14, wherein the correction control is carried out such that: when a rate of an absolute value of the difference in detected wheel speeds to the vehicle-body speed is larger, it is determined that a difference in diameter between the main driving wheels and the driven wheels is greater, and then the control threshold value is increased gradually; and when the rate of the absolute value of the difference to the vehicle-body speed is smaller, it is determined that the difference in diameter is smaller, and then the control threshold value is decreased gradually.
 16. The method as claimed in claim 14, wherein the control threshold value is calculated such that the vehicle-body speed is multiplied by an offset gain, and then divided by another vehicle-body speed upon calculation of the offset gain.
 17. The method as claimed in claim 16, wherein the correction control is carried out by setting the offset gain in accordance with a relationship between the difference in detected wheel speeds and the vehicle-body speed, wherein the control threshold value is increased gradually by increasing gradually the offset gain, and the control threshold value is decreased gradually by decreasing gradually the offset gain.
 18. The method as claimed in claim 13, wherein the traction distribution control is carried out by determining the engaging force of the clutch in accordance with a difference between the difference in detected wheel speeds and the control threshold value. 